1. Field of the Invention
The invention relates generally to the measurement of time varying electric fields, and more particularly to the measurement of electric fields with a frequency in the range of 1 MHz to 100 MHz.
2. Description of the Prior Art
Non-invasive sensing of the shallow subsurface is necessary for environmental monitoring and management, e.g. detection and delineation of buried hazardous wastes, and monitoring the condition of clay containment caps. Electromagnetic methods have been used for subsurface characterization, but there is a need for increased resolution for waste form characterization, verification, and monitoring activities. In particular, a compact detector which can be used on the surface or in a borehole is desired.
A window exists in the electromagnetic spectrum between ground penetrating radar (30 MHz to 1 GHz) and induction techniques ( less than 100 kHz) that has not been utilized for these applications. The frequency band of 1 MHz to 100 MHz is important for environmental work because of good earth penetration and good resolution. However, the frequency range between 1.0 to 100 MHz has not been used for existing electromagnetic or radar systems to detect small objects in the upper few meters of the ground. Ground penetrating radar (GPR) can be used successfully in this depth range if the ground is resistive but most soils are, in fact, conductive (0.01 to 1.0 S/m) rendering G.P. inefficient. For example, in a soil of 0.2 S/m the maximum range for a typical GPR is only 17 cm. Other factors controlling the resolution of GPR system for small objects is the spatial averaging inherent in the electric dipole antenna and the scattering caused by soil inhomogeneities of dimensions comparable to the wavelength (and antenna size). For maximum resolution it is desirable to use the highest frequencies but the scattering is large and target identification is poor. While a traditional radar approach could be used, the antenna length at these frequencies must be too long to be practical. Accordingly it is desirable to have a detector of electric fields in the 1-100 MHz frequency range.
A toroidal coil has been suggested as a transmitter if current is forced to flow through the winding (Wait, J. R., Excitation of a conducting half-space by a toroidal coil, IEEE Antennas and Propagation Magazine, Vol.37, No. 4, p.72-74, 1995). Generated within the toroid is a strong azimuthal magnetic field, which in turn can be considered equivalent to that of an electric dipole.
Accordingly it is an object of the invention to provide a simple and compact method and apparatus for detecting time varying electric fields, particularly in the frequency range of 1 MHz to 100 MHz.
The invention is a simple and compact method and apparatus for detecting high frequncy electric fields, particularly in the frequency range of 1 MHz to 100 MHz, using a toroidal antenna. For typical geophysical applications the sensor will be used to detect electric fields for a wide range of spectrum starting from about 1 MHz, in particular in the frequency range between 1 to 100 MHz, to detect small objects in the upper few meters of the ground.
Time-varying magnetic fields associated with time-varying electric fields induce an emf (voltage) in a toroidal coil. The electric field at the center of (and perpendicular to the plane of) the toroid is shown to be linearly related to this induced voltage. By measuring the voltage across a toroidal coil one can easily and accurately determine the electric field. The sensor will greatly simplify the cumbersome procedure involved with GPR measurements with its center frequency less than 100 MHz. The overall size of the toroidal sensor can be as small as a few inches. It is this size advantage that will not only allow easy fabrication and deployment of multi-component devices either on the surface or in a borehole, but it will render greatly improved resolution over conventional systems.